The present invention is directed to intermediates and a stepwise process for the conversion of D-methionine or certain of its derivatives to optically active 3-thiolanyl sulfonate esters of the formula ##STR2## wherein R is (C.sub.1 -C.sub.3)alkyl, phenyl or tolyl.
The compounds of formula (I) are particularly valuable intermediates in the preparation of certain penem antibiotics. Thus, antibacterial 5R, 6S-6-(1R-hydroxyethyl)-2-(cis-1-oxo-3-thiolanylthio)-2-penem-3-carboxylic acid, which is a diastereomeric mixture of two compounds, was earlier disclosed as a valuable antibacterial substance by Hamanaka, U.S. Pat. No. 4,619,924; while Volkmann et al., in European patent application No. 222397, have disclosed an improved synthesis for that substance. More recently, the preferred diastereoisomer [5R,6S-6-(1R-hydroxyethyl)-2-(1R-oxo-2S-thiolanylthio)-2-penem-3-carboxyli c acid] and a process therefor, have been identified by Volkmann in as yet unpublished International Application No. PCT/US87/01114, designating inter alia the United States of America. Key to Volkmann's process is an optically active intermediate of the formula (I), prepared by the following sequence: ##STR3## We have found that the optically active compounds of the formula (I) are more conveniently and readily prepared from D-methionine, or derivatives thereof.